1. Field of the Invention
The present invention relates to a heater driving device, for example, a heater driving device of thermal fixing device in an electrophotographic copying apparatus.
2. Description of the Related Arts
Conventionally, electrophotographic copying apparatus and the like comprise a hater driving device. Conventional heater driving device supplies alternating current (hereinafter AC) voltage to a heater, and conducts AC phase control or AC wave number control so as to control temperature. AC phase control turns on a heater every half cycle of AC voltage for a period between a predetermined phase angle and a zero cross point. While, AC wave number control controls supply of power to a heater in accordance with AC wave number present in a predetermined cycle. Thus, neither AC phase control nor AC wave number control can control supply of power to a heater in a shorter cycle, such as a cycle shorter than a half cycle of AC voltage.
On the other hand, a heater driving device is known which uses direct current (hereinafter DC) voltage. A heater driving device using DC voltage is capable of temperature control of great precision by modulating pulse width of DC voltage.
However, since power source for business is AC, a heater driving device using DC voltage necessitates a rectification circuit for converting AC voltage from the power source for business to DC voltage. However, provision of a rectification circuit brings about complication of circuit construction and enlargement of a device.